Report on surveys of reefs in the Townsville sector of the Great Barrier Reef

Summary

AIMS divers take time to ascend at the end of a dive. Trained professionals and volunteers combine to conduct field surveys as part of the Long Term Monitoring Program. For the volunteers it is a great opportunity to see the reef "expeditionary style" and contribute to research into how the Great Barrier Reef changes through time.

Outer-shelf reefs in the Townsville sector had a low coral cover due to Severe Cyclone Yasi (category 5) that passed to the north of this sector in February 2011. Surveys in 2013 indicated few signs of recovery since. This image taken on the flank of Chicken Reef is typical with barren reef framework swept clean of live coral and the benthic community dominated by a cover of coralline and turf algae.

As part of the Long Term Monitoring Program (LTMP), manta tows and intensive scuba surveys for benthic organisms, reef fishes, and agents of coral mortality were completed on seven reefs in the Townsville sector. Preliminary results of the manta tow surveys and scuba searches are reported here.

Median reef-wide live hard coral cover (henceforth coral cover) ranged from very low (0-5%) to moderate (10-30%) on reefs surveyed by manta tow. Coral cover had either remained stable or had increased since these reefs were last surveyed using manta tows. No crown-of-thorns starfish (COTS) outbreaks were recorded in this sector. A total of three COTS were seen on three mid shelf reefs (Rib, John Brewer and Davies) during broadscale manta tow surveys, and four COTS were seen on two mid-shelf reefs (John Brewer and Davies) during intensive scuba surveys. Only small numbers of feeding scars were seen on any of the reefs, so it is unlikely that high densities of COTS were present.

Occurrence of coral diseases was generally at or below background levels. Numbers of Drupella spp. (coral feeding snails) were also low, and at similar levels to previous years.

A summary of the results is presented in Tables 1 and 2. A full list of survey reefs appears in the Long-term Monitoring Status Report Number 8. Details of the monitoring program design, the sampling methods and a full explanation of the COTS outbreak terminology used in this report can be found on the AIMS reef monitoring website. A brief summary of the history of COTS populations and reefs affected on the GBR since 1986 can be found here.

Townsville sector reefs

The story is somewhat different for mid-shelf reefs in the Townsville sectors that were less exposed to the large oceanic swell generated by Severe Cyclone Yasi (category 5). Clear signs of recovery were recorded on all mid shelf reefs with Rib Reef in particular showing a rapid regrowth of hard corals on much of the reef. This photo was taken on the second flank of Rib Reef where coral cover was at its highest.

For the inshore reefs that were less exposed to Yasi than either mid- or outer-shelf reefs the story is even more complicated. This photo was taken on the flank of Havannah Island Reef that suffered from an extensive coral bleaching event in 1998. As result of bleaching the benthic community underwent a phase shift from being dominated by hard coral to being dominated by macro-algae. The benthic community remains dominated by macro-algae (i.e. has not recovered) some fifteen years after the initial disturbance.

Six reefs were surveyed using manta tow (Table 1) and seven were surveyed using intensive scuba searches (Table 2). All outer shelf reefs had low coral cover (0-10 %). These reefs are in the early stages of recovery after losses of live coral cover due to Severe Tropical Cyclone Yasi (category 5) that passed to the north of these reefs in February 2011. Coral cover on two mid-shelf reefs (Rib and John Brewer) was moderate (10-30%). Coral cover had increased on both these reefs since they were last surveyed in 2011. Rib Reef in particular has made a remarkable recovery with coral cover increasing from very low (0-5%) to moderate (20-30%). Coral cover at Davies Reef remained at a low level (0-10%), similar to that recorded in 2011.

Crown-of-thorns starfish were seen on three of the six reefs that were surveyed by manta tow. All three mid-shelf reefs had COTS at low levels: a single starfish was seen at each reef, meaning that populations were well below outbreak levels. Only a small number of feeding scars were seen, suggesting that current starfish densities were unlikely to be a threat to reef-wide live coral cover in the immediate future. No COTS were seen on any of the outer-shelf reefs that were surveyed.

Four COTS were recorded during scuba searches of the marked survey sites on two mid-shelf reefs, three at John Brewer Reef and one at Davies Reef. Scuba searches on the seven intensive survey reefs (Table 2) found low occurrences of coral diseases including "white syndrome" (WS, a disease-like necrosis found particularly on tabulate Acropora spp.), brown band disease (BrB) and "skeletal eroding band" (SEB), particularly on mid-shelf reefs. Virtually no cases of coral disease were seen on outer reefs, but coral cover was low. Davies Reef had the highest level of Skeletal Eroding Band disease, which was higher than the GBR long term average and higher than levels recorded in 2011. Drupella spp. (coral feeding snails) were present in numbers generally similar to those recorded when these reefs were last surveyed in 2011.

Table 1. Summary of results of manta tow surveys of six reefs in the Townsville sector